My experiments having been devoted especially to those branches of the subject which promised to be more immediately practically valuable, I have not pursued this inquiry very far, and offer it for your consideration as being not only interesting, but possibly worthy of full investigation.

GENERAL OBSERVATIONS ON THE PROPERTIES OF CELLS.

From the number of different properties possessed by my cells, it might be anticipated that the different combinations of those properties would result in cells having every variety of action.  This is found to be the case.  As a general rule, the cells are noteworthy in one respect only.  Thus, if a cell is extremely sensitive to light, it may not be specially remarkable in other respects.  As a matter of fact, however, the cells most sensitive to the light are also “U B cells.”

The property of sensitiveness to light is independent of the power to generate current by exposure to light—­the best current-generating cells being only very moderately sensitive to light, and some of the most sensitive cells generate scarcely any current at all.  Current-generating cells are, almost without exception, “U B cells;” and the best current-generating cells are strongly polarized, showing a considerable change of resistance by reversing the direction of a current through them; and they are also strong “anode cells,” i.e., the surface next to the gold offers a higher resistance to a battery current than the other surface of the selenium does.  The power to generate a current is temporarily weakened by sending a battery current through the cell while exposed to light, in either direction.  The current generated by exposure to light is also weakened by warming the cell, unless the cell is arranged for producing current by exposure to heat.

The properties of sensitiveness to light and to change of battery power are independent of each other, as I have cells which are sensitive to change of current but absolutely insensitive to light—­their resistance remaining exactly the same whether the cells are in darkness or in sunlight.  I also have cells which are sensitive to light, but are unaffected by change of battery power, or by reversing the direction of the current through them.

The sensitiveness to change of battery power is also independent of the sensitiveness to reversal of direction of the current.  Among the best “L B cells,” some are “anode cells” and others are “cathode cells,” while still others are absolutely insensitive to reversal of current or to the action of light.

Constancy of the resistance.—­A noticeable point in my cells is the remarkable constancy of the resistance in sunlight.  Allowing for differences in the temperature, the currents, and the light, at different times, the resistance of a cell in sunlight will remain practically constant during months of use and experiments, although during that time the treatments received may have varied the resistance in dark hundreds of thousands of ohms—­sometimes carrying it up, and at others carrying it down again, perhaps scores of times, until it is “matured,” or reaches the condition in which its resistance becomes constant.